Wavelength-division multiplexing (WDM) is a technique where light signals of different wavelengths are combined and simultaneously transmitted over a single optical fiber. Long distance WDM systems often use amplification to amplify the light signals.
One form of amplification is called Raman amplification. With Raman amplification, the amplification effect is achieved by an interaction between the signal and a pump laser within an optical fiber. One type of Raman amplification is commonly referred to as distributed Raman amplification. Distributed Raman amplification uses the transmission fiber as a gain medium by multiplexing a pump light wavelength with a signal wavelength. The pump light may be coupled into the transmission fiber in the same direction as the signal (co-propagating), in the opposite direction (counter-propagating) as the signal, or both.